What is a Main Sequence Star?
Stars begin their lives from clouds of dust and gases. A main sequence star is also born that way. The clouds are drawn together by gravity, forming a protostar.
A protostar is still gathering mass. It is not yet considered a star. Its core needs to be hot enough to support fusion. The temperature of the core must exceed 10 million K for fusion to happen.
Our Sun is an example of a main-sequence star. It is just one of the many. In fact, they make up about 90% of all the stars in stars that populate the universe. The mass of main-sequence stars ranges from about 0.10 to 200 times the solar mass.
Many objects undergo the same process of forming from dust and gases. However, not all of them become stars. Some objects become brown dwarfs. They are sometimes referred to as “failed stars” because they never ignite. Why is this so?
The mass of a brown dwarf is only 0.08 times the solar mass, or even less. If it has enough mass, the continued collapse of gas and dust will make it hotter. It will become hotter and hotter until it reaches the critical temperature to start the fusion process.
Unlike brown dwarfs, main sequence stars reach this stage thus, lighting up the skies.
Mass Effects Brightness
The mass of a star affects its luminosity. Since massive stars burn their fuel at a higher rate, they are also more luminous. That is why we can see a lot of main-sequence stars with the naked eye, except for the lower mass red dwarfs. Their color is also an indication of the star’s other characteristics. Blue-colored stars are hotter while red is cooler.
The brightness of stars and other celestial bodies was already measured in ancient times in terms of magnitude. A scale from one to six was used to determine their brightness. A lower number means a brighter celestial object and a higher magnitude indicates a fainter object. Stars were first categorized based on their brightness by the Greek astronomer Hipparchus.
Notable examples of main sequence stars are the Sun, Sirius A, Achernar, Alpha Centauri, and Altair.
Characteristics of a Main Sequence Star
Main sequence stars have different masses. The common characteristic they have is their source of energy. They burn fuel in their core through the process of fusing hydrogen atoms into helium.
The characteristics of main sequences stars, along with other stars, are categorized using different classification models. Examples are the Harvard Classification Scheme, Hertzsprung-Russell Diagram (HR Diagram), and the Morgan–Keenan (MK) System.
American astronomers Annie Jump Cannon and Edward C. Pickering made the Harvard Classification Scheme. They developed the system at Harvard college observatory.
They discovered that there was an overlap in the types A to Q previously identified in the Draper System. Seven letters are used in this classification. They are O-B-A-F-G-K-M. They are arranged in decreasing temperature. The blue-colored O class stars are the hottest while the M-type red stars are the coolest.
To easily remember this spectral sequence, the mnemonic “Oh Be A Fine Girl/Guy, Kiss Me” can be used.
Using the Harvard Classification Scheme, Danish chemist and astronomer Ejnar Hertzsprung observed that there are many stars in the K and M class.
Considering that they are either fainter or brighter than our Sun, they were grouped as “dwarf stars” and “giant stars.” They revealed a continuous and prominent sequence.
The main sequence is a continuous and distinctive band of stars. It is the most prominent aspect of the Hertzsprung-Russell (H-R) diagram as about 90% of the stars observed belong in this band. They are also plotted in terms of their color, brightness, and energy emission.
We can see that many stars are concentrated in the upper left part of the diagram. They are also hotter and brighter as indicated by their colors. The less numerous band of stars in the bottom right of the diagram shows cooler and fainter stars in the main sequence.
In the Morgan–Keenan (MK) System, stars are determined based on temperature and luminosity. It was based on the Harvard classification scheme, following the O, B, A, F, G, K, M spectral types. It uses Roman numerals together with the letters to add more information about a star. With this system, we can easily know if something is a dwarf, a giant, or a supergiant star.
Peculiar stars are also given their own classifications. Stars in the main sequence are assigned the Morgan-Keenan luminosity class V.
Life Cycle of a Main Sequence Star
A star is born when gas and dust collapse in their own gravity and fuse hydrogen atoms. They move away from the main sequence when they use up the hydrogen that fuels their cores.
Even though stars form around the same time in a star cluster, the length of their lifetime will still depend on their masses. The most massive of them will move away from the main sequence first. Low-mass stars will leave the main sequence last.
Red dwarf stars are an example of low-mass stars that stay for a long time in the main-sequence. When a red dwarf cannot sustain hydrogen burning anymore, it will sink and become a blue dwarf. It becomes more luminous as seen in the blue hue. It will then become a white dwarf, getting cooler over time and expel its outer envelope. At the end of its life, it will become the theoretical remnant called a black dwarf. No star has reached this stage yet.
Good Candidates For Supporting Life
Being the smallest stars in the galaxy, they will outlive all the others. They can live hundreds of billions or even trillions of years. That characteristic makes them good candidates in supporting life.
The Sun is bigger than a red dwarf so its life is shorter. Bigger stars have higher core temperatures. They need a larger amount of energy to continue burning. Because of this, they ran out of fuel earlier than less massive stars.
The Sun will remain a main-sequence star for about 10 billion years. That is still about five billion years considering our yellow star is 4.6 billion years old. Stars with about 10 times the mass of the Sun will only be in the main sequence for about 20 million years.
The course of stellar evolution depends on the mass of the star. After the main sequence stage they will either become red giants or red supergiants.
Stars of intermediate-mass enter the giant branch. When this happens, the helium core contracts, and a shell is formed around it. The outer layers of the star are expelled which results in a planetary nebula. What remains during this expulsion is the star’s core. It becomes a white dwarf. This is a sign that the star has reached the end of its stellar life. Red dwarfs do not pass through the red giant stage.
After the main sequence, stars with a high mass become red supergiants. The cores of these stars become so hot that helium and eventually heavier elements are fused together. At some point, their cores will eventually collapse, resulting in a bright supernova explosion. A neutron star or black hole will be left as a result of this explosion.
Main Sequence Star Features: Energy Generation and Transfer
Generally, astronomers divide the main sequence into two parts: the upper and lower parts. The two processes support nuclear reaction depending on the temperature in the star’s core region.
The proton-proton chain is primarily responsible for energy generation in the lower main sequence. Hydrogen is directly fused together to form helium in this nuclear fusion reaction. The lower limit to sustain this process equals the mass of 80 Jupiters.
The other reaction or the upper main sequence is known as the CNO cycle. The letters in its name stand for carbon, nitrogen, and oxygen. They serve as the intermediates that help fuse hydrogen into helium.
A main-sequence star like our Sun has three important parts. These are the core, the radiative zone, and the outer region.
Energy in a Sun-like star is transported because of the difference in temperature in the core and the surface. This is done through the process of radiation and convection.
The core makes up 25% of our Sun’s diameter. It is in a state of balance or hydrostatic equilibrium. This is achieved because the core’s outward pressure is met with the opposing inward pressure of the outer layers.
The radiation zone forms 70% of its diameter while the outer one is where convection happens. Plasma does not mix that much in the radiation zone as compared to where convection happens.
Convection carries energy through the movement of plasma. Everything in the outer region of the star is utilized, making it a more efficient process than radiation. This process happens because the hotter material moves upward while the cooler elements move downward.
Notable Main Sequence Stars
The Sun (Sol)
It is a main-sequence star which means that it generates energy through nuclear fusion in its core. It converts hydrogen into oxygen through this process. This conversion also generates solar radiation and subatomic particles called neutrinos.
The spectral type of our Sun is G2V. About 99.86% of the solar system’s mass is attributed directly to it. The diameter of this yellow star is about 1.39 million kilometers (864,000 miles). It is more massive than our planet, Earth, by about 330,000 times. Its surface temperature is around 5,778 K.
The apparent magnitude of the Sun is −26.74. It is the brightest object that we can see in our sky, considering its distance. Light travels at about 8 minutes and 19 seconds from the Sun’s horizon to the horizon of our planet. This light is responsible for the existence of life here on Earth. It drives photosynthesis, as well as the changes in our weather.
There will be a time when the Sun can no longer continue the fusion of hydrogen. Its temperature will increase and so does its size. Eventually, it will become a red giant star.
When this happens, the Sun will consume almost everything around it. The planets that will be affected most are the closest ones to it, Mercury and Venus. The Earth will also be scorched. That is expected after five billion years though because the Sun is still halfway through its life in the main sequence.
Sirius is commonly known as the “Dog Star.” It is the brightest star in the constellation of Canis Major. Aside from that, it is also the brightest star that we can see in our night sky.
Sirius is a binary star. Its primary component, Sirius A, is a main-sequence star. The fainter companion is a white dwarf, designated Sirius B. Their orbital period is 50 years.
Sirius A is an Am star. Its apparent magnitude is −1.46, much brighter than its companion star. This A-type star has a mass of about two Suns. It has a surface temperature of 9,940K, radiating 25 times the luminosity of the Sun.
The Sirius star system is located close to the Sun, at a distance of about 8.6 light-years. That is also one of the reasons why it looks so bright from our perspective.
Achernar is in the “the river” of the constellation of Eridanus. This main-sequence star is the brighter component of the Alpha Eridani star system. The system has an apparent magnitude of 0.40 – 0.46. It ranks as the ninth brightest star in our night sky.
It is a very hot star which is evident in its blue color. Achernar is also a fast spinner, which results in its oblate shape.
This B-type star is nearly seven times as massive as the Sun. it is also much more luminous, radiating at approximately 3,150 solar luminosities. There is a difference in temperature between the areas of this star because of its shape. Its average temperature is 15,000 K.
Alpha Centauri is a triple star system. Its components are Rigil Kentaurus, Toliman, and the Proxima Centauri. They are all in the main sequence. It is the closest star and planetary system to us. It lies at a distance of only 4.37 light-years
The main component, Rigil Kentaurus, is a Sun-like star. Its stellar classification is G2V. it is very bright as an individual star, with an apparent magnitude of −0.01. It is more massive than the Sun by roughly 10%.
Toliman is an orange star with the stellar classification of K1V. Proxima Centauri is the third member of the system. This small star is a red dwarf. It is of M6 Ve spectral type.
Altair is a bright main-sequence star. It has an apparent magnitude of 0.77. Its spectral type is A7 V. It is 1.8 times as massive as the Sun and it is 11 times more luminous. Its high rotation rate affected its shape.
Main Sequence Stars: Additional Facts
The Orion Nebula is a famous natal cloud that gives birth to many main-sequence stars. This deep-sky object is in the Milky Way. It is visible to the naked eye because it has an apparent magnitude of +4.0.
Some call main sequence stars “dwarf stars.” This poses confusion because, by definition, a dwarf star is a star relatively small in size. It is not necessarily lesser in luminosity. Even though the majority of main sequence stars are dwarf stars, not all of them are in the main sequence.
Aside from its long life, there are still many other factors that challenge the ability of red dwarfs to support life. This includes the possibility of the planet being tidally locked. Stellar spots, radiation, and flares are other considerations.
- What is the main sequence?
- How do main-sequence stars generate their energy?
- What is the factor that determines the lifespan of a main-sequence star?
- What will happen when a star moves away from the main sequence?
- Name some examples of main-sequence stars?